There is almost no danger in Shoshone, so the key is this:
Seems to have decent idea of whats going on, and is very aggresive.
I say definitely take him on Shoshone. If he's gung ho, a swim won't demoralize him and will instead challenge him and show him what he needs to work on. Doing a run like this is such a big high for newbies. This is a great run.
But, it is also a good idea of taking him down Grizzley Creek the first time and seeing how he does. Leave it to him.
I think the biggest danger is if he runs it successfully, he'll have too much confidence and will urge him to try something harder that has danger.
If he runs Shoshone successfully the first time, take him again and make him play in the waves so he flips and learns his limits.
You should have a tow rope and he should have warm gear he can swim in.
Tell him that if he flips, he shouldn't rush his roll. He should let the boat stabilize and let a few waves pass before he starts his roll.
Also make sure he pauses in an eddy after each rapid to take a break.